(1) Field of the Invention
The invention relates to a process for fabricating a photomask, and more particularly, to a process for fabricating an alternating phase-shifting mask and alignment monitor.
(2) Description of the Prior Art
As photolithography advances to 0.35 microns and below, new technologies are required to increase resolution of the imaging lens. One new technology is the use of alternating phase-shifting photomasks. Phase shift masks take advantage of the interference effect in a coherent or partially coherent imaging system to reduce the spatial frequency of a given object, to enhance its edge contrast, or both. It is possible to control locally the type of interference, destructive or constructive, at critical locations in a design by adding an additional patterned layer of transmitting material on the mask. This technique results in a combination of higher resolution, larger exposure latitude, and larger depth-of-focus. In phase shift lithography, a transparent coating is placed over a transparent area. The light waves passing through the coated region are delayed 180xc2x0 C. out of phase with the light waves passing through the uncoated region. At the edge of a phase-shifted area, the light waves from the phase-shifted and clear areas will cancel out producing a more sharply defined interface.
The primary mask pattern consists of a patterned opaque material, such as chromium, overlying a transparent substrate, such as quartz. The phase-shifting pattern is formed by etching grooves into the transparent material at the edges of the opaque pattern or by overlaying a patterned transparent material over the opaque pattern. It is essential that the opaque pattern and the phase-shifting pattern be aligned to each other. Thus, an alignment monitor is also fabricated as part of the phase-shifting mask in order to determine if the mask has been fabricated with proper alignment.
U.S. Pat. No. 5,756,235 to Kim discloses a phase-shifting mask having an alignment monitor. The alignment monitor is formed in a space between dense patterns of the mask. FIG. 1A illustrates in top view Kim""s APSM and alignment monitor. FIG. 1B illustrates cross-sectional view 1xe2x80x941 across FIG. 1A. The chrome pattern 12 has been formed on the transparent substrate 10. The patterned phase-shifting material 14 overlies the chrome pattern. The phase-shifting pattern 16 in the space between the chrome pattern is Kim""s alignment monitor. Kim""s alignment monitor cannot be implemented on the most common type of PSM which is formed by etching grooves into the transparent material at the edges of the opaque pattern. Also, using Kim""s alignment monitor it is difficult to tell in which direction the misalignment occurs.
Other patents disclose various alternating phase-shifting masks and methods of aligning the masks in steppers. U.S. Pat. No. 5,897,979 to Tzu et al describes a APSM having alignment marks to align a stepper. U.S. Pat. No. 5,604,059 TO Imura et al shows an APSM. U.S. Pat. No. 5,780,188 to Rolson teaches a method of step and repeat exposure of a mask pattern. U.S. Pat. No. 5,786,116 to Rolfson discloses fabrication of an ATOM mask using alignment marks to align the primary mask pattern and the grating pattern. U.S. Pat. No. 5,538,819 to DeMarco et al discloses a formed mask having reinforced alignment marks to be used to align the stepper.
Accordingly, it is a principal object of the present invention to provide an effective means for determining misalignment between the phase-shifter and the opaque pattern in the fabrication of an alternating phase-shifting mask.
Another object of the invention is to provide a process for fabricating an alternating phase-shifting photomask having an alignment monitor.
Yet another object of the present invention is to provide a process for fabricating an alternating phase-shifting photomask having an alignment monitor wherein the alignment monitor can determine misalignment between the phase-shifter and the opaque pattern of the alternating phase-shifting mask.
Yet another object of the present invention is to provide a process for fabricating an alternating phase-shifting photomask having an alignment monitor wherein the alignment monitor can determine misalignment and the direction of misalignment between the phase-shifter and the opaque pattern of the alternating phase-shifting mask.
In accordance with the objects of this invention a new process for fabricating an alternating phase-shifting photomask having an alignment monitor is achieved. An opaque layer is provided overlying a substrate. The opaque layer is patterned to provide a mask pattern. A phase-shifting pattern is formed on the substrate wherein a portion of the phase-shifting pattern comprises an alignment monitor whereby alignment between the mask pattern and the phase-shifting pattern can be tested.